For example, I think that if a big basketball star wanted to have such a clause in his contract, the NBA League and the star's basketball team would not want to lose him and therefore would give in. This is a contract demand that does not require extra money or extra resources on the part of the team or the league so to deny it and potentially lose a great player would be an unpopular move.
The top of the top guys in the world always have that drive, and will step up to any challenge. Injuries can occur anywhere, yet players still play in the Olympics, in summer ball, and in other international tournaments. I could see somebody like Lebron having a similar clause, as he could beat a number of different types of players in different ways.
Whether Michael Jordon is the greatest basketball player of all time or not is debatable, but he is undoubtedly the most recognizable and will probably always be noted as the person who brought the most attention to the game. He inspired millions and was allowed a lot of privileges that are not available to others.
Yes, it is reasonable to allow NBA players to play whoever they want despite the risk for injury. If these contracts are allowed to dictate certain activities as being off-limits, we are on a slippery slope. Why not ban players from driving sports cars and motorcycles? Or any car for that matter? Mandatory protective gear when showering, to minimize injury if they slip? Risk management must always be weighed against common sense and the well-being of the players.